A powder paint is increasingly demanded in a large number of industrial fields of automobiles, domestic electric appliances, furniture, machine tools, business machines, building materials, toys, etc. as a low-pollution paint using no organic solvent. When the powder paint is applied to a metallic finish containing a flake pigment such as an aluminum pigment, however, it is so difficult to arrange the flake pigments in parallel with a substrate that color tone is darkened and no sufficient metallicity is obtained. In order to overcome this disadvantage of the powder metallic pigment composition, considerable efforts for research and development have been made in various fields.
Conventionally developed methods of manufacturing powder metallic paints include a melt blending method of sufficiently kneading and mixing metal flake pigments in advance with resin or a coloring pigment by a melt process and thereafter pulverizing the same by crushing or the like, a dry blending method of mixing resin powders and flaky pigments with each other and applying the mixture, a bonded method using resin powders containing metal flaky pigments bonded to surfaces thereof, and the like (see, for example, Japanese Patent Laying-Open No. 51-137725 (Patent Document 1), Japanese Patent Publication No. 57-035214 (Japanese Patent Laying-Open No. 50-087426, Patent Document 2), Japanese Patent Laying-Open No. 09-071734 (Patent Document 3), and U.S. Pat. No. 4,138,511 (Patent Document 4)).
In the melt blending method, however, the flake pigments readily deform through the kneading and mixing step or a subsequent step of adjusting a particle size of the resin powders by crushing or the like. Therefore, appearance of a film obtained by applying a powder metallic paint manufactured with this method is not sufficiently satisfactory. When the metallic pigment is prepared from aluminum particles in this manufacturing method, active surfaces of aluminum are disadvantageously exposed in the crushing step to increase danger of ignition, dust explosion or the like.
The dry blending method has such an advantage that the metallic pigment is relatively less likely to deform, however, the metallic pigment must be charged in application when the powder paint is applied with electrostatic coating using a corona method (a high-voltage forced charging method), and hence the surface of the metallic pigment must be coated with resin in advance if such a metal pigment as aluminum particles is employed as the metallic pigment. Further, the resin powders and the metallic pigment easily separate from each other in application due to different charging rates between the metallic pigment and the resin powders. Therefore, the content of the metallic pigments in the powder paint varies before and after application and hence the color tone so changes when the paint is recovered and used that it is virtually impossible to recycle the paint.
The corona method is a charging method, in which air passing in the vicinity of an electrode is charged, charge transfer from the charged air to resin powders or pigment particles occurs, and consequently the resin powders or pigment particles are charged. Generally, charge transfer from air to a powder paint is less than 1%, and the charged powder paint and a large amount of air bearing charges of the same conductivity type (generally negative charges) arrive at a plate to be painted. Here, in the case of a powder metallic paint, such a phenomenon that flake pigments that have once electrostatically been deposited are partially blown away due to electrostatic repulsion by wind bearing charges of the same conductivity type occurs, and consequently the film after baking suffers color unevenness. In addition, in an example where dry-blended metallic pigments are applied with the corona method, the problem of spitting is inevitable, although a degree may be different.
Meanwhile, there is another triboelectrification-type (also referred to as tribo-type) coating method available as a method of electrostatic powder coating. In triboelectrification, unlike the corona method, resin powders or pigment particles are directly rubbed against each other and charged, so that no charged air is generated. Therefore, in applying a dry-blended powder paint containing metallic pigments, no color unevenness is caused. In addition, as a high-voltage electrode does not exist, spitting is also less likely. Triboelectrification, however, achieves charging by rubbing resin powders or pigment particles against each other, and hence it also has such shortcomings that a raw material having triboelectrification characteristics (characteristics that a material is charged by friction; also referred to as tribo-characteristics) can only be used. A raw material poor in triboelectrification characteristics can be used for triboelectrification-type coating after a positive charging agent is added to the powder paint as an additive. In general, alumina fine particles (such as aerogel of alumina), titanium oxide and the like are used, however, there are also raw materials that cannot have triboelectrification characteristics sufficient for coating even after a positive charging agent is added, and such a raw material cannot be used for triboelectrification-type coating.
Other than the melt blending method and the dry blending method, a bonded method with which a flake pigment is bonded to resin powders is available. A method of adhering a metallic pigment to a resin powder surface with a brush polisher, a method of transferring and bonding a metallic pigment to resin powders by bringing the resin powders into contact with a dispersion medium such as an alumina ball covered with the metallic pigments, and the like have conventionally been proposed as the bonded method. Recently, however, it seems that the following two methods have generally been established. One is a method of heat sealing of a powder paint resin and a metallic pigment by dispersing the powder paint resin and the metallic pigment using a three-dimensional rotary mixer or the like at a temperature around a melting point of the powder paint resin (H. Groebl, “Farbe and Lacke,” 80, 930 (1974)). Another method is a method of bonding a powder paint resin and a metallic pigment to each other using a binder, in which they are dry-blended in a dispersion tank, thereafter a binder solution is sprayed while stirring and dispersion is continued, and a binder solvent is subsequently distilled by producing vacuum in the dispersion tank (see WO02/094950 (Patent Document 5)). These methods are advantageous in that a rate of introduction of the metallic pigment in the film is stable, color unevenness is less likely, and the powder paint recovered without adhering to a substrate can be reused.
With the heat sealing method, however, it is extremely difficult to evenly and uniformly heat powders in mass production facilities and hence a resin in a local portion where a temperature is excessively high is molten and blocking occurs. Therefore, an operation is difficult. Namely, it is difficult to bond the whole metallic pigments to the resin powders and hence free particles of the metallic pigments not bonded to the resin powders also remain in a large quantity.
If the quantity of free particles of the metallic pigments is increased, a blending ratio between the resin powders and the metallic pigments changes due to difference in bonding efficiency when the paint is recovered and used, and the paint cannot be reused after recovery as in the dry blending method. When a metal pigment such as aluminum particles or the like is employed as the metallic pigment, danger of ignition, dust explosion or the like is also increased due to a large quantity of free metallic pigments.
In any case, though the bonded method is an excellent method in the sense of recovery and reuse of the paint that has not deposited to the substrate and improvement in color unevenness, from a point of view of sufficient metallicity, it is equivalent to the dry blending method, although it is superior to the melt blending method. The greatest disadvantage of the bonded method is cost. Any method belonging to the bonded method is a process starting from dry blending, and when the bonded method is compared with dry blending in terms of cost, every cost for the process leads to increase in cost.
From a point of view of improvement in metallicity, a technique related to scaly aluminum having a surface covered with a phosphate-group-containing compound is also disclosed (see, for example, Japanese Patent Laying-Open No. 2001-029877 (Patent Document 6)). The document above describing a phosphate-group-containing unsaturated monomer and styrene, however, also states that a hydroxyl-group-containing unsaturated monomer is essential. Protons of hydroxyl groups are also active protons, and complicated chain transfer reaction occurs, reproducibility of a composition of a generated polymer or the like cannot be achieved, and performance becomes unstable when polymerization is performed under coexistence of the phosphate-group-containing unsaturated monomer with the hydroxyl-group-containing unsaturated monomer due to polymerization under existence of two types of active protons different in properties. Further, this document is silent about a fluoric monomer.
In addition, though the document above describes a polymer of a phosphate-group-containing unsaturated monomer and perfluorocyclohexyl(meth)acrylate which is a fluoric monomer, it is a dispersion prepared by dispersing polymerizing particles with a macromolecular dispersion stabilizer that is described in this document, and the polymer is not dissolved in a solution. When covering with such a dispersion is conducted, in a covering process, an adsorbed macromolecular dispersion stabilizer layer forms an outermost layer regardless of whether micellar particulates aggregate on scaly aluminum to cover the same or micellar rupture occurs to form a film, and hence effects of the invention are significantly affected by the macromolecular dispersion stabilizer.
In order to solve these problems, the present inventors invented a method employing an aluminum flake pigment having a surface covered with fluororesin containing alkyl fluoride group in molecules, and filed a patent application (see, for example, Japanese Patent Laying-Open No. 2003-213157 (Patent Document 7)). This method can achieve arrangement of aluminum flake pigments in parallel with a substrate by causing the aluminum flake pigments to float at the surface of a film utilizing low affinity of alkyl fluoride group to other substances and attains excellent metallicity despite powder coating.
On the other hand, adhesiveness is so disadvantageously poor due to low affinity of alkyl fluoride group to other substances that an overcoat layer tends to peel off when an overcoat clear is applied or a mending layer tends to peel off when a flaw or the like of the film is mended with the same paint.
As a result of further dedicated studies conducted in order to solve this problem, the present inventors have found that a flake pigment suitably used in a powder paint and providing high brightness to a film and good adhesiveness can be obtained by covering a surface of a flake particle with a coat having a resin composition containing a copolymer including a bond unit originating from a fluoric polymerizable monomer having fluoroalkyl group and a bond unit originating from a polymerizable monomer having phosphate group, and filed a patent application (see WO2005/007755 (Patent Document 8)). According to this invention, in powder coating as well, metallicity as sufficient as that of a solvent-based or water-based paint could be obtained.
So long as the powder paint manufactured with the dry blending method is applied with the corona method, however, even an excellent pigment as such cannot solve the above-described problems specific to corona, and color unevenness is caused.    Patent Document 1: Japanese Patent Laying-Open No. 51-137725    Patent Document 2: Japanese Patent Publication No. 57-035214    Patent Document 3: Japanese Patent Laying-Open No. 09-071734    Patent Document 4: Specification of U.S. Pat. No. 4,138,511    Patent Document 5: WO02/094950    Patent Document 6: Japanese Patent Laying-Open No. 2001-029877    Patent Document 7: Japanese Patent Laying-Open No. 2003-213157    Patent Document 8: WO2005/007755